This invention relates to a catalyst for reacting SO2 with molecular oxygen to form SO3, and also to a process of producing sulfuric acid from SO3 and water, where the SO3 is produced catalytically by reacting SO2 with molecular oxygen.
The production of sulfuric acid from SO2, which first of all is catalytically oxidized to form SO3, is described in detail in Ullmann""s Encyclopedia of Industrial Chemistry, 5th edition, vol. A25, pages 644 to 664. The known catalysts for the oxidation of SO2, which contain for instance V2O5 as active component, preferably operate at a temperature in the range from 380 to 620xc2x0 C. Higher temperatures will damage the catalyst. This leads to the fact that the gas supplied to the catalysis should have an SO2 content of not more than about 12 vol-%, so that the exothermicity of the oxidation reaction can easily be controlled. The DE-C-27 10 350 describes a catalyst for the conversion of SO2 into SO3, which operates at a temperature in the range from 600 to 800xc2x0 C. The catalyst has a silicon oxide carrier with a tridymite structure and an active component containing iron, copper and an alkali metal.
It is the object underlying the invention to create a catalyst suitable for continuous operation, whose activity and stability are also ensured at temperatures of 700xc2x0 C. and above. Furthermore, the catalyst should form the basis for a process of producing sulfuric acid, where gases with a high concentration of SO2 are used.
In accordance with the invention, the catalyst comprises a porous carrier and an active component connected with the carrier, where the active component consists of 10 to 80 wt-% iron, the carrier has a BET surface of 100 to 2000 m2/g and an SiO2 content of at least 90 wt-%, and the weight ratio carrier:active component lies in the range from 1:1 to 100:1. As carrier materials there may be used silicates, in particular zeolites (e.g. zeolites of the beta type), mesoporous silica gels (e.g. zeolites of the beta type), mesoporous silica gels (e.g. synthesized in accordance with U.S. Pat. No. 3,556,725 or MCM-41 of Mobil Oil as pure SiO2 material), also those mesoporous silica gels with up to 10 wt-% foreign elements (e.g. boron), diatomaceous earth, amorphous SiO2 or mesoporous alumosilicate (e.g. aluminum-containing MCM-41 of Mobile Oil). Advantageous carriers comprise for instance 90 to 100 wt-% of a zeolite or mesoporous SIO2. Details concerning the mesoporous SiO2 can also be found in WO-A-91/11390 and in xe2x80x9cMicroporous Materialsxe2x80x9d 10 (1997), pages 283-286.
The iron-containing active component of the catalyst may in particular consist of at least 80 wt-% iron oxides. The active component may in addition contain sodium, potassium and/or cesium. The content of these alkali metals may be up to 10 wt-%, based on the total weight of the catalyst.
The active component of the catalyst may furthermore include vanadium and/or sulfur compounds (e.g. pyrosulfate). In the active component, the weight ratio V:Fe may lie in the range from 1:1 to 1.3:1. For the sulfate content in the catalyst, 1 to 7 wt-% are recommended, based on the total weight of the catalyst. When the active component of the catalyst also contains copper, the Cu content will be up to 1 wt-% of the Fe content.
In the laboratory, the following catalysts were prepared:
First Catalyst
As starting material, there was used a mesoporous SiO2 with an ordered pore structure, with amorphous walls and a pore system with a regular hexagonal array with pore sizes between 2 and 8 nm (synthesized in accordance with WO-A-91/11390). It has a good thermal stability up to 1000xc2x0 C. and a BET surface of about 1000 m2/g. To 10 g of an aqueous 25% C16H33N(CH3)3Cl solution a mixture of 1.8 g soda waterglass (composition: 27.5 wt-% SiO2, 8.3 wt-% Na2O, plus water), 1.3 g SiO2 and 10 g water was added within 15 minutes. After stirring for 30 minutes, the suspension was heated for 48 hours in a screwed polypropylene vessel to a temperature of 90xc2x0 C. Then, it was filtrated, washed and dried for 8 hours at 90xc2x0 C. The dried mixture was heated to 550xc2x0 C. with a heating rate of 1xc2x0 C. per minute and maintained at this temperature for 5 hours. 1 g of this product was thoroughly mixed with 3.5 ml of a 0.95 mol Fe(NO3) solution and subsequently dried for 2.5 hours a 90xc2x0 C. The product was stirred for one hour in 25 g distilled water, filtrated, dried at 90xc2x0 C. and then thermally treated as follows: heating to 400xc2x0 C. with a rise of 5xc2x0 C. per minute, maintaining at 400xc2x0 C. for 3 hours, then heating to 700xc2x0 C. with a rise of 5xc2x0 C. per minute, and subsequently maintaining at this temperature for 3 hours. The product had a BET surface of 478 m2/g, the weight ratio Si:Fe was 5:1.
Second Catalyst
3 g commercial SiO2 (BASF D11-10) were added to a solution of 0.18 g NH4VO3 in 20 ml water. Then, 0.62 g FE(NO3)3 9H2O, dissolved in 1 g water, were added dropwise by stirring quickly. The product was filtrated, washed, dried, heated to 800xc2x0 C. and maintained at 800xc2x0 C. for 24 hours. The weight ratio Si:Fe:V is 33:1:1.3. In the same way, iron vanadate can be applied onto carriers with a large surface.
Third Catalyst
Here, a zeolite-like iron silicate (structural type beta-zeolite) is used as carrier material; the iron silicate has a three-dimensional system of micropores and has a large BET surface of 600 m2/g. A first aqueous solution was prepared as follows: 78.5 g 40% tetraethylammonium hydroxide and 10.7 g 40% hydrogen fluoride were added to 260.4 g tetraethyl orthosilicate in a polypropylene vessel. 70% of this solution were separate, and to the remaining 30% of the first solution 3.6 g FeCl3, dissolved in 9 g water, were added by stirring. Finally, there were added 22.2 g NH4F to the previously separated solution. The preparation was heated for 24 hours at 70xc2x0 C. in the open vessel, and the dry gel was subsequently dissolved in 10 g water. Upon inoculation with nuclei (beta-zeolite, 5 wt-%) the product crystallized in the course of 15 days in the polytetrafluoro-ethylene vessel at 170xc2x0 C. The product was heated to 200xc2x0 C. of 2xc2x0 per minute, maintained at this temperature for 3 hours, then heated to 550xc2x0 C. of 5xc2x0 C. per minute, and maintained at this temperature for 10 hours. The elemental analysis of the product revealed an atomic composition of H:Si:Fe:O:F= 104:60:4.3:178:0.4.
Samples of the three catalysts described above were tested in the laboratory, so as to determine their activity with respect to the oxidation of SO2 to form SO3. Of each catalyst, 0.5 ml of a fraction with particle sizes between 500 to 1000 xcexcm were maintained in the nitrogen stream for three hours at 324xc2x0 C. for activation purposes. For measuring the activity, 24.7 ml/min of a gas consisting of 20 vol-% SO2, 22 vol-% 02, and 58 vol-% N2 were passed over the activated catalyst samples, where a dwell time of 1.2 s in the catalyst bed was obtained. The activity (percentage of the converted SO2) in dependence on the temperature is indicated in the following Table.
The catalysts in accordance with the invention are particularly suited as precontact, so as to partly convert a gas with a high content of SO2 into SO3 and produce sulfuric acid, before the residual gas with a reduced content of SO2 can be passed for instance into a conventional production of sulfuric acid. The gas containing SO2 and O2 with an SO2 content of 13 to 50 vol-% and an oxygen content corresponding to O2/SO2 volume ratio of at least 1:2 is supplied to a precontact stage, in the precontact stage the gas and the oxygen are passed through at least one bed (precontact bed) of a granular catalyst (precontact), where the precontact has the features described above, and the maximum temperature at the precontact is maintained in the range from 580 to 800xc2x0xc2x0C. In the precontact stage 20 to 80% of the supplied SO2 are converted to SO3, and from the precontact stage a first gas mixture containing SO3 is withdrawn, which is cooled to temperatures of 50 to 300xc2x0 C. andis passed into at least one absorber, where in the absorber the first gas mixture is brought in direct contact with circulating sulfuric acid containing water, and a partial stream of sulfuric acid is withdrawn. From the absorber, a second gas mixture containing SO2 is withdrawn, is heated to a temperature of 380 to 600xc2x0 C. and with an SO2 concentration of 10 to 30 vol-% is introduced into a subsequent oxidation stage, in which SO2 is catalytically reacted with oxygen at temperatures of 480 to 770xc2x0 C. to form SO3. In further process steps, the SO3 produced in the oxidation stage is processed to obtain sulfuric acid. In the subsequent oxidation stage, usual catalysts are used. These catalysts may have active components, which for instance consist of at least 5 wt-% V2O5.
The precontact stage may for instance have at least two pre-contact beds, through which the gas flows one after the other. Expediently, the gas containing SO2, O2 and SO3 is cooled between the precontact beds to temperatures of not more than 550xc2x0 C. From the last precontact bed a gas is withdrawn after an intermediate absorption with preferably not more than 13 vol-% SO2 and is introduced into a subsequent oxidation stage.
A flow diagram of the process will now be explained with reference to the drawing.
A gas rich in SO2, to which O2-containing gas (e.g. air) has been admixed through line 3, is supplied to the precontact stage 1 through line 2. The SO2 content of the gas in line 2 lies in the range from 13 to 50 vol-% and mostly at least 15 vol-%, and the gas has preferably been preheated to temperatures of 350 to 500xc2x0 C. In the process variant represented in the drawing, the precontact stage 1 consists of the fixed bed 4 of the temperature-resistant catalyst, which here is referred to as precontact; the bed is referred to as precontact bed 4. It may be expedient to provide at the entrance of the bed 4 a conventional catalyst (e.g. vanadium contact) in a thin layer as a so-called ignition layer, in order to sufficiently increase the temperature in the gas, so that the oxidation reaction in the bed of the precontact will fully start immediately.
At the entrance of the precontact bed 4 an O2/SO2 volume ratio of at least 0.5:1 is ensured. At the precontact, a rise in temperature is effected by the oxidation reactions during the formation of SO3. A first SO3-containing gas mixture leaves the precontact stage 1 via line 6 with temperatures in the range from 580 to 800xc2x0 C. and preferably 600 to 700xc2x0 C. This first gas mixture is cooled in the waste heat boiler 7 to temperatures of 50 to 300xc2x0 C., where a valuable high-pressure steam may be recovered from water. The gas mixture then enters a first absorber 9, which is designed like a Venturi scrubber. Sulfuric acid coming from line 10 is sprayed into the gas, where the concentration of the sulfuric acid is increased by the absorption of SO3. The sulfuric acid formed in the first absorber 9 flows through line 11 to a collecting tank 12, excess sulfuric acid, whose concentration usually lies in the range from 95 to 100 wt-%, is withdrawn via line 13.
From the collecting tank 12, through the circulating pump 15 and line 16, sulfuric acid is supplied to the first absorber 9 and also to a second absorber 14, which is connected with the first absorber by the passage 17. SO3-containing gas flows through the passage 17 to the second absorber 14 and there upwards through a layer 19 of contact elements, which is sprayed with sulfuric acid from line 10a. Water is supplied via line 20, and the sulfuric acid discharged via line 21 likewise flows into the collecting tank 12. In practice, the absorbers 9 and 14 may also be designed differently than represented in the drawing.
The gas flowing upwards in the second absorber 14 releases sulfuric acid droplets in the droplet separator 24, and then flows through line 25 to a heater 26, which raises the temperature of the gas to 380 to 500xc2x0 C. The gas in line 27, here also referred to as second gas mixture, has an SO2 concentration of 10 to 30 vol-%. Due to this relatively low SO2 concentration, it may be supplied to a conventional sulfuric acid plant 28, which employs the usual catalysts for oxidizing SO2 to form SO3. The mode of operation and the structure of such a conventional plant is known and described for instance in Ullmann""s Encyclopedia of Industrial Chemistry, as mentioned above.